The present invention relates to a control stage for a high temperature steam turbine and particularly relates to buckets of the control stage having internal cavities within the airfoils of the buckets to reduce the weight of the bucket and creep damage in the turbine rotor.
The control stage of a steam turbine, i.e., the first stage of the turbine downstream of the control valves and steam inlets, creates unique loading on the control stage buckets. Typically, there are four inlets for admitting steam to the control stage, with a control valve for each inlet. The inlets are generally arranged in quadrants. Consequently, steam is emitted over segments of arcs and is therefore not uniformly applied on the first stage bucket airfoils. Because of this unique non-uniform flow and, hence, loading on the control stage, and over long-term operation at high temperatures, creep damage can and does occur in the turbine rotor, particularly in the coupling between the buckets and the rotor.
Typically, the buckets are secured to the rotor by dovetails. For example, the buckets may have first are second dovetails extending in an axial direction. Alternate dovetails on the rotor are received between the first and second bucket dovetails. The remaining rotor dovetails are received between the bucket dovetails of next-adjacent bucket. The rotor is typically formed of a material which does not have the high creep or rupture strength characteristic of the creep or rupture strength of the buckets end, accordingly, creep damage can and does occur about the rotor dovetails. Conventionally, the dovetails of the bucket and rotor are secured to one another by insertion of axially extending crush pins in openings formed in both the bucket and rotor dovetails. Creep strains around the pinholes of the dovetails, particularly in neck regions of the rotor dovetails, have been demonstrated. Accordingly, there is a need to control the loads or stresses in those regions to prevent creep strain from causing cracks to develop in the rotor dovetail.